Lime slaking method

ABSTRACT

An improved lime slaking system including agitator means effective for mechanical comminution of particulate lime during the hydration thereof.

This application is a continuation of application Ser. No. 200,811,filed Jan. 27, 1980, which is a division of application Ser. No. 16,260,filed Feb. 28, 1979, now both abandoned.

In the art of lime slaking or hydration as heretofore known andpracticed various types of apparatus and methods have been utilized toprovide hydrated lime which is used, for example, in wet or dry flue gasdesulfurization systems such as are commonly associated with coalburning electric power generating stations or other fossil fueledprocess plants. The slaking process is the exothermic hydration reactionof water with calcined lime (quicklime). Often in the art "slaking" hasbeen used to refer to a process utilizing large amounts of excess waterto distinguish from hydration processes wherein only sufficient water isused; however, for purposes of the following disclosure the termshydration and slaking are used interchangably.

Examples of known lime slaking systems include U.S. Pat. No. 1,204,699,which discloses a system including an elongated reaction vessel and aseries of oppositely pitched agitator blades which rotate slowly tocontrol flow of the quicklime and water mass through the vessel. OtherU.S. patents including U.S. Pat. Nos. 845,190; 1,565,107; 1,679,149;2,888,324 and 3,066,016 typify prior lime slaking system in which thequicklime and water mass is contained in a reaction vessel and is gentlyagitated during the hydration process by one of various stirring ormixing means such as plural blades or paddles, a rotating drum, or rakeand bowl structures to maintain a uniform mix of quicklime and water orto slowly advance the mix through the reaction vessel as the hydrationreaction proceeds. Many prior lime slaking systems, such as in the abovementioned U.S. Pat. Nos. 845,190 and 1,679,149, have required grindingor milling of the quicklime input as by passing it through a millingdevice such as a ball mill or a hammer mill prior to its being added tothe reaction vessel, and this is often referred to as 100 percentgrinding of the quicklime.

Another variety of known lime slaking system comprises a milling devicesuch as a ball mill which is in closed circuit with a classifierapparatus, for example a spiral classifier. In this system the quicklimeand water are passed as a stream through the ball mill for 100 percentgrinding to form a lime slurry, and the mill fulfills two purposes asfollows: first, it is operative as a comminution device for sizereduction of the calcined lime particles and particulate impurities, andsecond, it is a reaction chamber wherein the hydration reactionproceeds. The slaked lime is directed to the spiral classifier forseparation of particulate impurities such as silica and residual,oversized quicklime particles from the slaked lime and the separatedparticulates and residual quicklime are then recycled to the ball mill.Inasmuch as the ball mill in such systems functions as a reactionchamber, very little lime hydration occurs in the spiral classifieritself.

Although these and other prior lime slaking systems have generallyserved their intended purposes, they have nevertheless often beensubject to various shortcomings. For example, in prior systems requiringa mill for 100 percent grinding of the quicklime prior to its additionto a reaction vessel, the milling device has been a relatively largecost factor in the system due to the relatively high cost of millingcapacity. This is particularly true of the last mentioned slaking systemin which the mill must be of sufficient capacity to be utilized both asa comminution device and as a reaction chamber in which the hydrationreaction may proceed virtually to completion in order to minimize thequantity of unreacted quicklime in the mill output. Thus, the mill oftenmay be not only cost inefficient as a capital expenditure but inaddition may be a process bottleneck due to its limitations as areaction vessel.

Of course, lime slaking without initial 100 percent grinding of thequicklime is known, but this approach often has resulted in relativelylarge amounts of residual, unreacted quicklime in the hydrated limemass, and thus has been undesirably wasteful of increasingly moreexpensive materials.

As noted hereinabove, many prior lime slaking systems have includedmeans for gentle agitation of the lime and water mass within a reactionvessel; however, such agitators have served only to maintain a uniformmix of lime and water or to slowly advance the mixture through thereaction vessel in a controlled manner. For example, U.S. Pat. Nos.1,204,699 and 2,888,324 disclose the use of agitator speed reductionmeans to provide gentle agitation of the lime and water mass. Accordingto these patents, the agitators slowly sweep the entire cross sectionalarea of the reaction vessel to uniformly advance the lime and water massthrough the reaction vessel. Other known agitators are intended toprovide for uniform distribution of the water throughout the lime mass,as disclosed in U.S. Pat. No. 803,506 for example. Known lime slakingagitation devices thus have not materially enhanced system throughputcapacity inasmuch as they have not alleviated those conditions whichtend to inhibit the hydration reaction rate. For example, in the priorart, the common mechanism of hydration has included rapid hydration ofthe pellet exterior surface followed by more gradual hydration ofinterior pellet portions with a consequent heating of the pellets untilthe individual pellets explode. This mechanism is less predictible andproduces a less uniform reaction than a receding surface mechanismwherein the hydrated lime pellet surface would be removed as it forms.Conventional lime slaking system agitators have not been able to removehydrated surface portions from the quicklime pellets in significantenough quantities to materially enhance the hydration process.

It may be noted here that effective quicklime particle comminution isnot provided by mere intensity of agitation. Rather, the specificmechanics of the agitation are the determining factor. To be effectivefor quicklime particle comminution an agitator must promote highintensity rubbing or abrasion of quicklime particles in the lime masswhereby the abrasion of the particles will be effective to remove theslaked lime surface therefrom. The mere intensification of known limeslaking system agitation processes, if effected, would not provide sucha receding surface mechanism inasmuch as the number of high intensityparticle contacts would not be significantly increased thereby, otherfactors being equal. Thus, increased mechanical energy input to priorart agitators would have produced no significant benefit. In addition,it is noted that the prior art teachings of gentle or mild agitation areconsistent with concerns that anything which might promote asubstantially higher reaction rate must be approached with caution inthat it is conceivable under such circumstances that an uncontrollabletemperature and reaction rate excursion could occur.

Other related shortcomings of prior practice and teaching include therequirement in some cases to add large amounts of excess water to thequicklime for purposes of temperature control as taught by U.S. Pat. No.2,888,324, for example. This produces a low density of lime solids inthe water and dissipates heat needed to sustain the hydration reaction.This and the resulting requirement for system capacity sufficient toseparate large excess volumes of water from the slaked lime adverselyinfluence system production capacity.

These and other shortcomings of prior lime slaking systems and practiceare overcome by the present invention according to which there isprovided a lime slaking method and apparatus including the use of anattrition scrubbing apparatus as a reaction chamber means wherein thecontents of the scrubber vessel follow a progressive course of movementunder the impetus of associated and cooperative concurrent andcountercurrent impelling influences with a high degree of particleattrition deriving from the opposed impelling influences. In its use forlime slaking, the agitators of the attrition scrubbing apparatus providevigorous countercurrent flow of a lime and water mass of high solidsconcentration to cause the quicklime pellets to abrade against oneanother as the hydration reaction proceeds thus promoting an effectivereceding surface reaction mechanism whereby the slaked lime surface ofthe pellets is continuously removed to expose unreacted pellet portionsto the water for continued, uniform reaction until the hydrationreaction has proceeded virtually to 100 percent completion. Theinvention obviates the need for preliminary milling of the quicklimepellets and can tolerate a wide variation in particulate impurities inthe lime and water mix, quicklime pellet size, and other processvariables.

The present invention may further include a residuals processing systemportion including a classifier and a relatively small mill such as aball mill connected such that the hydrated lime is fed to the residualsprocessing portion of the system from the attrition scrubber forprocessing of unreacted quicklime and particle impurities.

Some advantages of the invention include:

1. Easy maintenance of a sufficiently high reaction vessel temperaturein the attrition scrubber due to the high net energy input from theconnected impeller horsepower and the high solids concentration in thelime mass.

2. 100 percent grinding of the quicklime is not required and the smallermill requirement therefore offers a capital cost savings. In some casesthe mill may be eliminated entirely thereby reducing system maintenancerequirements;

3. The lime slaking process can be controlled at a higher density ofquicklime in water and over a wider range of densities. Thus the systemcan be made more responsive to changes in slaking requirements;

4. The capital investment in the system is lower than for heretoforeknown systems of comparable production capacity.

5. The receding surface reaction mechanism provides a more uniform andeasily controlled reaction even at higher reaction rates than commonlythought to be advisable heretofore.

These and other objects and advantages of the invention are more fullyspecified in the following description with reference to theaccompanying figures in which:

FIG. 1 is a partially schematic illustration of a lime slaking systemconstructed and operable in accord with one embodiment of the instantinvention; and

FIG. 2 is a partial schematic system illustration similar to FIG. 1illustrating an alternate embodiment of the invention.

There is generally indicated at 10 in FIG. 1 a lime slaking systemconstructed and operable according to one embodiment of the instantinvention and including an attrition scrubbing apparatus 12 having afeed box 14, a discharge box 16, and a plurality of cells 18a, 18bcommunicating sequentially in fluid flow relation and arranged such thatmaterial feed deposited in feed box 14 is directed through an inlet 20into the first cell 18a and is then processed sequentially through thecells 18a, 18b to a discharge box outlet 22. Each cell 18a, 18b includesa vessel 24 having associated therewith an agitator means 26 forinducing a countercurrent flow within the material contained in therespective vessel 24. Agitator means 26 is shown as incorporating a pairof spaced impellers 28a, 28b of opposite pitch and of different flowinducement capacity by virtue of differing degree of pitch or differingimpeller diameters, or both. The opposing pitch of impellers 28a, 28binduces the desired countercurrent material flow between the respectiveimpeller pairs to provide in each cell 18a, 18b an attrition scrubbingaction whereby high intensity rubbing and abrasion between particles ofthe contained material provides for efficient comminution or sizereduction of the feed material particles. The differing impeller flowcapacities are provided to induce a net flow of feed material from theinlet of each respective cell 18a, 18b toward the outlet thereof asindicated by arrows A in the figures. The countercurrent flow ofmaterial is flow counter to the flow direction of arrows A as indicatedby arrows B. Agitators 26 thus operate in concert to provide the desiredattrition scrubbing action in each cell 18a, 18b and in addition provideuniform flow sequentially through the cells 18a, 18b from feed box 14.In operation, impeller 28a directs material in descending flow towardthe bottom of vessel 24 and impeller 28b is arranged to interrupt theflow and direct a substantial portion of the material countercurrent tothe descending flow. As impeller 28a has a greater output capacity, acontinuous or progressive descending flow from inlet 20 toward theoutlet of cell 18a is maintained, but intense attrition and scouringoccur in the zone of countercurrent flow.

During particle travel through each cell 18a, 18b, the movement thereofthrough the countercurrent flow zone between the impellers 28a, 28bserves to bring each particle into a multitude of contacts with otherparticle surfaces, and a high concentration of solids in the vesselcontents insures ample attrition over the entire particle surface toprovidde a thorough scouring. The change in direction of flow insuccessive cells, as shown, assists in bringing new particle surfacesinto rubbing contact. Inasmuch as the attrition scrubbing apparatus 12per se, as described hereinabove, is known and forms no part of theinstant invention, further detailed description thereof is notconsidered necessary. For more detailed description of attritionscrubber apparatus 12 the reader is referred to U.S. Pat. No. 3,054,230,which issued Sept. 18, 1962, to L. H. Logue, and the entire disclosureof which is incorporated herein and made a part hereof by reference.

Outlet 22 of Discharge box 16 communicates in fluid flow relation as byany suitable conduit means 30 with a classifying apparatus such as aspiral classifier 32 for delivery of the attrition scrubber outputthereto. In a pool area of classifier 32 particles larger than apredetermined size are separated from the remainder of the containedmaterial by spiral rake means 34 and the separated particles aredirected by way of any suitable conveying means, shown schematically at36, to a milling device such as a ball mill 38 for grinding thereof. Theground particles are separated from waste materials in a separator means40 which may be a screen separator portion of ball mill 38, for example,and are recycled to the pool area of classifier 32 for furtherprocessing. The waste material from separator means 40 is directed toany suitable waste disposal facility 42.

All material input to classifier 32 which is smaller than thepredetermined size for separation remains in the pool area of classifer32 and ultimately is directed as by overflowing a weir or dam (notshown) into a storage facility such as a holding tank 44.

In the context of the present invention the respective vessels 24 ofattrition scrubbing apparatus 12 provide reaction chamber means tocontain a hydration process of quicklime and water while spiralclassifier 32, ball mill 38 and separator means 40 provide forprocessing of any residual solids larger than a predetermined size.These may include quicklime residuals and particulate contaminantsremaining after virtual 100 percent completion of the lime hydrationprocess in attrition scrubber 12. Accordingly, the flow of feed materialmay be constituted by quicklime particles or formed pellets and water,these components being directed together or separately into feed box 14of attrition scrubber 12 to form a mixture in the proportion of, forexample, approximately 25 to 30 percent solids concentration. Ifdesired, feed box 14 may be utilized for the input of quicklime onlyinto a quicklime and water mass contained within the first cell 18a ofscrubber 12 with additional water being added as needed directly intocell 18a. For operation of the system each cell 18a, 18b is filled withquicklime and water mixture, and the mixture is subjected to a vigorouscountercurrent flow attrition scrubbing action induced by the respectiveagitator means 26 in each cell 18a, 18b and indicated by oppositelydirected arrows A and B, whereby the abrasion of quicklime pelletsagainst one another is effective to produce a receding surface reactionmechanism and the continuously forming hydrated lime surface iscontinuously removed from the pellets as fresh, unreacted quicklime isexposed to the water for further hydration. Thus, as a result of themechanics of the agitation process and the use of a much higher thanusual lime solids concentration, the hydration reaction may proceed at agreater rate than it would without the benefit of these innovations andwithout need of 100 percent grinding. In addition, agitators 26, byvirtue of differing flow capacities of their impellers 28a, 28b, alsoinduce a net flow of the lime and water mass sequentially from one cellto the next as shown by the arrows A generally indicating the path offlow.

Of course, it will be appreciated that inasmuch as attrition scrubber 12is shown partially broken away, additional scrubber cells similar in allrespects to the illustrated cells 18a, 18b may be included in theintervening space between cells 18a, 18b. By the judicious selection ofattrition scrubber cell size, impeller capacities and other systemdesign parameters including lime slurry density, the hydration reactionmay be carried virtually to 100 percent completion in attrition scrubber12 such that the system output is substantially 100 percent hydrated orslaked lime with only small residual quantities of unreacted quicklimeremaining. To separate these residuals and any particulate contaminants,for example silica contaminants, from the slaked lime mass the dischargeflow from outlet 22 is directed by way of conduit 30 to the pool area ofspiral classifer 32 wherein the hydrated lime mass is separated from thequicklime residuals and particulate contaminants. The separatedquicklime residuals and contaminants are directed to ball mill 38 foradditional processing while the hydrated lime mass continuouslyoverflows the weir in the spiral classifier pool and is directed intoholding tank 44 for eventual use as desired. The quicklime residuals andparticulate contaminants are milled in mill 38. Milling of the quicklimeresiduals tends to promote further hydration whereby the lime residualsare reduced to fines of smaller size than the ground contaminantparticles. Thus, the milled material is processed through separatormeans 40 with the lime residuals being returned to the pool area ofclassifier 32 for additional processing and other particulates beingdirected to waste disposal facility 42.

It will be appreciated that although a certain minimal amount of limehydration occurs in the ball mill 38 and classifier 32, it isnevertheless an object of the invention that the vast majority of thehydration of lime pellets occur in attrition scrubbing apparatus 12whereas classifier 32 and ball mill 38 function primarily as separationand comminution devices. Therefore, a far smaller milling capacity isrequired for the described system than in many prior systems in which100 percent grinding of the quicklime input to the reaction vessel wasrequired, or in those prior systems in which the mill itself is thereaction vessel. The attrition scrubbing apparatus 12, by providing arelatively violet countercurrent flow condition in a plurality of cells,promotes an efficient, uniform lime slaking reaction in a mannersuperior to heretofore known lime slaking systems and without requiringmilling capacity equivalent to the total mass transfer capacity of thesystem.

In FIG. 2 there is shown an alternate embodiment of the instantinvention in which the discharge flow of hydrated lime from dischargebox 16 may be directed in parallel via conduits 45a, 45b to a respectivepair of spiral classifiers 46a, 46b, each of which is in closed circuitvia conduits 47a, 47b with a single ball mill 48 for processing of theseparated particulate contaminant and quicklime residuals as in theembodiment of FIG. 1. The separator portion of ball mill 48 feeds groundlime residuals via conduits 50a, 50b to each of spiral classifiers 46a,46b, and feeds waste byproducts to waste disposal facility 52. Theparallel connected spiral classifiers 46a, 46b may feed the hydratedlime via conduits 54a, 54b to a common holding tank 56 as shown, or maydirect their separate outputs to different storage facilities (notshown) for use in different processes. Depending upon the desiredprocess parameters, spiral classifiers 46a, 46b may be of differentsize, capacity, separation capability, and so forth to provide differinggrades of hydrated lime.

Additionally shown in FIG. 2 is a branch feed line 58 whereby a feedflow of quicklime or quicklime and water mix may be directed partiallyto feed box 14 of attrition scrubbing apparatus 12 via branch 58a andpartially to ball mill 48 via branch 58b. This alternative would providesuch advantages as permitting closer control of the water content in thehydrated lime mass by the selective addition directly into ball mill 48of quicklime which, along with the quicklime residuals from classifiers46a, 46b, would be ground and recycled to spiral classifiers 46a, 46b.In this way the net amount of unhydrated residual lime recycled toclassifiers 46a, 46b may be selectively increased or decreased tocompensate for an excess or shortfall of water. In a similar embodiment(not shown) branch 58b may feed into any part of the system downstreamfrom the last cell 18b and may feed water as an alternative to feedingthe lime and water mix or lime alone.

The lime slaking system shown in FIGS. 1 and 2 may include various typesof classifying and milling devices of widely varying capacitiesdepending upon the quality of the quicklime and water input to thesystem and the quality requirements of the slaked lime output. Thus, ifthe attrition scrubbing process alone, with proper input material,produces slaked lime having a residual quicklime and contaminant contentwhich is consistant with requirements of the end use of the slaked lime,then the residuals classifying and grinding operations may beeliminated. In general, however, some residuals processing capabilitynormally will be required to economically produce slaked lime ofacceptable quality.

According to the description hereinabove there is provided by theinstant invention a novel apparatus and method for lime slaking whereinvigorous countercurrent flow agitation is applied to a quicklime andwater mass to provide a lime slaking system of greater efficiency andeconomy than prior systems. The invention minimizes the need forgrinding, milling and classifying apparatus by providing apparatus inwhich the hydration process can be carried out with greater efficiencyand uniformity than has been heretofore possible in known apparatus ofcomparable cost and capacity. In addition the method of the inventionprovides for a lime slaking process not available in heretofore knownlime slaking apparatus.

The invention is not intended to be limited to those specificembodiments described hereinabove, but rather may be subject to variousmodifications without departing from the broad spirit and scope thereof.For example, the specific mechanical design parameters of the describedattrition scrubber, classifiers and mills may be varied within a widerange of values, and the specific arrangement of the residualsprocessing portion of the system may include various known processingunits in various system structural arrangements in lieu of or inaddition to the illustrated arrangement. These and other embodiments andmodifications having been envisioned and anticipated by the inventor,the invention is intended to be interpreted as broadly as permitted bythe scope of the claims appended hereto.

What is claimed is:
 1. The method of producing hydrated lime comprisingthe steps of: providing a mass of unhydrated lime particles and water;contacting exterior surface portions of said lime particles with saidwater to convert said exterior surface portions into hydrated lime;coincident with said converting of said exterior surface portions ofsaid lime particles, agitating said mass between an impeller directingsaid mass in descending flow and another impeller arranged to interruptsaid descending flow and direct a substantial portion of said masscountercurrent to said descending flow so as to promote high intensityinterparticle contact among said lime particles; consequent to saidpromoting high intensity interparticle contact, mechanically removingsaid hydrated exterior surface portions from said lime particles toexpose unhydrated internal portions thereof to said water; andcontinuing said contacting, agitating, mechanically removing andexposing until substantially all of said lime particles have beenconverted substantially entirely into hydrated lime.
 2. The method asclaimed in claim 1 wherein said step of providing a mass of unhydratedlime particles and water includes providing a mass of unhydrated limeand water at a concentration of greater than approximately twenty-fivepercent solids.
 3. The method as claimed in claim 1 including theadditional steps of passing said hydrated lime through a classifer forclassifying thereof into a first relatively fine component and a secondrelatively coarse component, mechanically reducing said secondcomponent, and admixing at least a portion of said second component withsaid hydrated lime.
 4. The method of claim 1 wherein said agitation stepis effective to provide energy input to drive the hydration reaction forproducing hydrated lime at a reaction rate greater than the spontaneoushydration rate of said lime particles.
 5. The method of claim 1 furthercomprising the step of processing said hydrated lime through aclassifier to classify said hydrated lime into a hydrated lime componentand a residual component.
 6. The method of claim 5 further comprisingthe step of processing said residual component through a comminutiondevice to reduce said residual component.
 7. The method of claim 6further comprising the step of recycling at least a portion of saidreduced residual component to said classifier.
 8. A method for producinghydrated lime of a quality useful for flue gas desulfurization,comprising: directing unhydrated lime particles and water between animpeller directing said particles and water in descending flow andanother impeller arranged to interrupt said descending flow and direct asubstantial portion of said particles and water counter-current to saiddescending flow so as to contact exterior surface portions of said limeparticles with water to convert said exterior surface portions intohydrated lime; driving said impellers so as to subject said particles tovigorous counter-current flow to promote high intensity interparticlecontact among said lime particles; and consequent to said driving topromote high intensity interparticle contact, removing said hydratedexterior surface portions to expose unhydrated interior portions of saidlime particles to said water to convert said exposed interior portionsinto hydrated lime.
 9. The method of claim 8 wherein said directing stepcomprises directing unhydrated lime particles and water between saidimpellers at a concentration greater than twenty-five percent solids.10. A method for reacting unhydrated lime particles and water to producehydrated lime, comprising: mixing said unhydrated lime particles andliquid water to create a slurry and to contact exterior surface portionsof said lime particles with said water to convert said exterior surfaceportions into hydrated lime; directing said slurry among pluralcounter-current flow inducing impellers including an impeller directingsaid slurry in descending flow and another impeller interrupting saiddescending flow and directing a substantial portion of said slurrycounter-current to said descending flow; adding energy to said slurry todrive said reaction at a rate greater than the spontaneous hydrationrate of said lime particles solely by driving said counter-current flowinducing impellers to promote high intensity interparticle contact amongsaid lime particles in said slurry; and consequent to said energyaddition and interparticle contact, removing said hydrated exteriorsurface portions to expose unhydrated interior portions of said limeparticles to said water.
 11. The method of claim 10 wherein saiddirecting to promote counter-current flow, adding energy to produce highintensity contact, removing of said exterior surface portions and saidexposing of unhydrated interior portions are continued untilsubstantially all of said lime particles have been convertedsubstantially entirely into hydrated lime.
 12. The method of claim 11wherein said continuation until substantially all of said lime particleshave been converted comprises directing said unhydrated interiorportions, said water and said removed hydrated exterior surface portionsserially among plural sets of plural counter-current flow inducingimpellers.
 13. The method of claim 10 wherein said directing stepcomprises directing said slurry among said counter-current flow inducingimpellers at a concentration of solids greater than approximatelytwenty-five percent.